Remove popup asking for preferred language on websites

Here's the popup, which displays after switching away from the default language. Pardon my french (screenshot).

We suspect it's confusing and most users will click it away anyway, and maybe will take the wrong choice. Research in tor-browser#40414 (closed) seems to agree about it.

Moreover, websites aren't reliably using that hint to serve localized content to users and are as likely to base it on IP location.

For these reasons We would like to remove that popup and keep the sane default.

It is a blocker to have multi-language support in Mullvad Browser.